How can one of my players learn feyspeak/fiendspeak?

My warlock recently acquired a new magic item, not realizing it was cursed. He is now afflicted with the inability to knowingly tell a direct lie. The issue I hadn't anticipated, however, is that the art of deception by way of spoken truth is not as universal as I'd assumed, and said player is asking where he can learn it. I very sadly do not have the time to instill in him the particulat parental traumas that taught me, so does anyone know of any sorts of guides in how to craft your words to avoid telling lies whilst avoiding sharing the truth?